On Fri, 24 Mar 2006, Per Jessen wrote:
All,
probably a little OT here, but I _am_ using SUSE Linux :-)
I'm trying to work out the optimal (or near-optimal at least) RAID configuration of a 24-disk array. The array comes with two redundant RAID controllers each with six SCSI channels. These controllers allow all kinds of RAID0/1/5 configurations, but no RAID6.
I want the array to be able to survive a two disk failure, so RAID6 would be the obvious choice, but ...
So I'm sort of looking at choosing between -
- using plain software RAID6 and ignoring the hardware RAID facilities of the array. - using the hardware controllers to build a combination of RAID0/1/5 that'll give me the two drive failure survivability.
I've had a look around the web, but googling for "two drive failure RAID" almost always leads to someone talking about RAID6 ...
So, opinions/suggestions?
Go here:
http://en.wikipedia.org/wiki/RAID5
It's really very good.
Remember with raid levels you read them from left-to-right, so, raid 10
is "raid 1 and then raid 0" meaning N arrays of raid1 which are combined
to form a raid0. So raid 50 would be N raid5 arrays combined together
in a (single) raid0.
I prefer Linux's software raid to most hardware raid - I've experienced
several problems with hardware raid but not with software raid, and the
talk about hardware raid being faster isn't backed up with statistics or
facts.
Raid is always a trade-off between space and reliability (and to a
lesser extent, speed - these days a very modest machine can easily do
software raid5 calculations faster than the hardware can deliver the
data ( on my Duron 750 the raid5 checksum speed is 2.7GB/s ). Choose
your tiering such that you can tolerate multiple drive failures at
multiple levels for maximum redundancy.
Raid 55 using only for 9 disks, for example gives you 3 raid 5's
combined to form another raid5.
You can lose /one drive/ from 2/3 of the lower-level raid5's *and* /all/
of the drives from the 3rd lower-level raid5 and still be OK.
Using raid5 with more than 3 disks and you start to raise your risk
factor a bit because 3 disks minimises the number of disks you have and
therefore the number that are likely to fail. Adding more disks raises
the likelihood of failure /without/ reducing the ability to /tolerate/
failure - adding more disks to a raid5 only adds space.
With 24 disks you could go with a fairly complicated but almost
unbelievably robust setup, and in fact with 24 disks if you can afford
the space you can mix some mirroring up in there.
--
Carpe diem - Seize the day.
Carp in denim - There's a fish in my pants!
Jon Nelson